Cerium oxide and zirconium oxide are known compounds that are particularly useful constituents, either alone or in combination, in a wide variety of catalyst compositions, e.g., multifunctional catalyst compositions, especially catalysts suited for the treatment or conversion of exhaust gases emanating from internal combustion engines. By "multi-functional" is intended a catalyst capable of effecting not only the oxidation, in particular, of carbon monoxide and of hydrocarbons present in the exhaust gas, but also the reduction of the oxides of nitrogen also present in such gas ("three-way" catalysts).
To meet stringent air emissions regulations, catalysts are being placed closer and closer to the engine thus subjecting them to higher temperatures. In order to maintain their effectiveness, they should not sinter and loose surface area. Consequently, there exists a need for catalysts that maintain high surface area even after prolonged exposure to temperatures of 900.degree. C.
Further, when a catalyst structure consists of a mixture of various catalytic components, such as ceria and zirconia, it follows that a more intimate mixture of the components will result in a more effective catalyst structure.
Attempts to prepare mixed oxides of cerium and zirconium having a high and thermally stable surface area are known. For example, U.S. Pat. No. 5,717,218 discloses a thermally-stable, high surface area ceria-zirconia mixed oxide having a pure monophasic CeO.sub.2 crystalline habit. The mixed oxides are prepared by subjecting a mixture of oxide solutions to thermohydrolysis, preferably in a nitrogen atmosphere and under pressure, to form the desired oxide. This process is undesirable since it is time consuming and requires expensive equipment such as high pressure reactors.
U.S. Pat. No. 5,532,198 also discloses a process of preparing a high surface area cerium/zirconium mixed oxide. The oxides are prepared by admixing a zirconium sol with a cerium sol, spray drying the admixture and calcining the dried material. The process requires that the ratio of the mean size of the zirconium sol particle to the mean size of the cerium sol be within a specified range in order to obtain a product having a sufficiently high surface area.
It has also been proposed in Japanese Patent Application No. (Kokai) 55,315/1992 to prepare fine powders of cerium oxide and zirconium oxide having a high specific surface area and excellent heat resistance by a coprecipitation process. The powders are prepared by mixing a water-soluble zirconium salt with a water soluble salt of cerium (III) or cerium (IV) to form a mixed salt solution, and thereafter treating the salt solution with excess base to precipitate a mixed oxide powder. Where the cerium salt is a cerium (III) salt, the Japanese reference teaches adding hydrogen peroxide to the salt solution simultaneous or subsequently with the precipitation step to oxidize the trivalent cerium to the tetravelant state. The highest surface area reported for cerium/zirconium powders produced in accordance with the process disclosed in this reference was only 26.5 m.sup.2 /g.